Recently, mobile communications terminals, electronic appliances, etc. having various functions and high performance have been getting required to be smaller and lighter, so that electronic parts are disposed in a narrower space at higher density, with their speeds increasing. Accordingly, among circuits and parts, electromagnetic wave noises, particularly high-frequency noises have become serious problems. To suppress such near-field electromagnetic wave noises, various noise suppression sheets have been proposed and put into practical use.
Many of such noise suppression sheets contain magnetic materials and/or conductive materials. For example, JP 2010-153542 A discloses an electromagnetic wave noise suppression sheet comprising a substrate, a conductive layer formed by an electrically conductive coating material containing particles, flakes or thin wires of metals such as Cu, or carbon, and a magnetic layer formed by a magnetic coating material containing soft-magnetic materials such as ferrite, Sendust, Permalloy, etc. JP 2006-278433 A discloses a composite sheet for suppressing electromagnetic wave noises, which is obtained by laminating two or more calendered sheets each comprising soft-magnetic powder such as amorphous flakes having a composition of Febal—Cu1—Si12.5—Nb3—Cr1—B12 (atomic %), for example, and a resin, and further calendering the resultant laminate for integration. However, any of the noise suppression sheets disclosed in JP 2010-153542 A and JP 2006-278433 A does not have sufficient capability of absorbing near-field noises, is difficult to be made thinner because it contains magnetic materials and/or conductive materials blended in the resin, and suffers a high production cost.
JP 2006-279912 A discloses a sputtered thin film of AlO, CoAlO, CoSiO, etc., as a thin film for suppressing near-field electromagnetic wave noises generated in a quasi-microwave band, which has surface resistance controlled to 10-1000 Ω/square matching to the characteristic impedance Z (377Ω) of free space, to have a reflection coefficient (S11) of −10 dB or less and a noise suppression effect (ΔPloss/Pin) of 0.5 or more. However, this thin film for suppressing near-field electromagnetic wave noises does not have sufficient electromagnetic wave absorbability.
JP 2008-53383 A discloses a radiowave-absorbing and shielding film having excellent heat dissipation characteristics, which comprises a graphite film having different thermal conductivities in plane and thickness directions, and a soft-magnetic layer formed on the graphite film, which contains soft-magnetic materials such as Fe, Co, FeSi, FeNi, FeCo, FeSiAl, FeCrSi, FeBSiC, etc., ferrite such as Mn—Zn ferrite, Ba—Fe ferrite, Ni—Zn ferrite, etc., or carbon particles. However, this radiowave-absorbing and shielding film does not have sufficient electromagnetic wave absorbability.
JP 2006-93414 A discloses a conduction noise suppression body comprising a conduction noise suppression layer of 0.005-0.3 μm in thickness formed by a physical vapor deposition method on a substrate made of plastics such as polyesters, etc., which may contain powder of a soft-magnetic metal, carbon, ferrite, etc., the conduction noise suppression layer being made of at least one soft-magnetic metal selected from the group consisting of iron, cobalt and nickel, and the conduction noise suppression layer comprising portions having a crystal lattice in which soft-magnetic metal atoms are arranged with intervals of several angstroms, extremely small portions composed of plastics without the soft-magnetic metal, and portions in which the soft-magnetic metal is dispersed without crystallization in the plastics. However, because the conduction noise suppression layer in this conduction noise suppression body is a single layer, its thickness is difficult to control. Accordingly, in most Examples, the soft-magnetic metal is blended in the plastic substrate. In Example 4, which is an only example using a plastic substrate containing no soft-magnetic metal, the power loss ratio (Ploss/Pin) at 1 GHz is as small as 0.55.